FIG. 10 is a cross-sectional view of a prior art semiconductor light emitting device. In FIG. 10, reference numeral 1 designates an n type (hereinafter, referred to as `n-`) GaAs substrate. An n-Al.sub.0.35 Ga.sub.0.65 As first cladding layer 2 is disposed on the n-GaAs substrate 1. A p type (hereinafter, referred to as `p-`) Al.sub.0.06 Ga.sub.0.94 As active layer 3 is disposed on the n-Al.sub.0.35 Ga.sub.0.65 As first cladding layer 2. A p-Al.sub.0.35 Ga.sub.0.65 As second cladding layer 4 is disposed on the p-Al.sub.0.06 Ga.sub.0.94 As active layer 3. An n-Al.sub.0.10 Ga.sub.0.90 As contact layer 5 is disposed on the p-Al.sub.0.35 Ga.sub.0.65 As second cladding layer 4. A contact hole 7 is produced at the center of the contact layer 5. The contact layer 5 is doped to produce a Zn diffused p.sup.+ region 6, the diffusion depth of which is controlled such that the diffusion front reaches into the p-Al.sub.0.35 Ga.sub.0.65 As second cladding layer 4 at a region under the contact hole 7 while it remains within the n-Al.sub.0.10 Ga.sub.0.90 As contact layer 5 at the other regions.
A description is given of the operation.
A bias is applied between the Zn diffused p.sup.+ region 6 of the n-Al.sub.0.10 Ga.sub.0.90 As contact layer 5 and the n-GaAs substrate 1 with the former as positive side. No current flows at regions where the contact hole 7 is not produced because a pnpn junction is present on the device, while a current flows at the region where the contact hole 7 is produced because the n-Al.sub.0.10 Ga.sub.0.90 As contact layer 5 is converted into the p.sup.+ region due to the Zn diffusion and only a pn junction is formed between the p-Al.sub.0.06 Ga.sub.0.94 As active layer 3 and the n-Al.sub.0.35 Ga.sub.0.65 As first cladding layer 2 which are biased by the bias voltage in a forward direction. Thus, the current flows along the current path 8 shown in FIG. 10. Holes and electrons injected into the p-Al.sub.0.06 Ga.sub.0.94 As active layer 3 due to the current 8 recombine and radiate light. The light has an energy corresponding to the energy band gap of the material constituting the active layer. For example, when the active layer is constituted by Al.sub.0.06 Ga.sub.0.94 As, the peak wavelength of the light is approximately 830 nm.
In the prior art semiconductor light emitting device, light generated in the p-Al.sub.0.06 Ga.sub.0.94 As active layer 3 and spreading in a direction toward the substrate excites the n-GaAs substrate 1 and electron-hole pairs are generated in the n-GaAs substrate 1. Since the n-GaAs substrate is generally produced by a horizontal Bridgman (HB) method, it cannot have a high dopant concentration and it is usually doped at a concentration of around 1.about.3.times.10.sup.18 cm.sup.-3. In this carrier concentration, the intensity of photoluminescence (hereinafter, referred to as PL) light generated due to the band to band transitions is strong. Accordingly, the electron-hole pairs are excited in the n-GaAs substrate 1 by the light spreading from the p-Al.sub.0.06 Ga.sub.0.94 As active layer 3 in a direction toward the substrate and recombine in the substrate, so that the band to band transitions of GaAs occur and generate PL light at a wavelength of 870 nm. As a result, light generated in the active layer 3 with the PL light superposed thereon is emitted from the device. FIG. 11 shows a spectrum of the light emitted from the device. As shown in FIG. 11, the spectrum has a main peak at 830 nm followed by a sub peak of 870 nm. The semiconductor light emitting device having such spectrum cannot be used for the optical communication including multiple wavelengths, such as three wavelengths of 830 nm, 850 nm, and 870 nm, because interference arises therebetween.
FIG. 12 is a cross-sectional view illustrating a construction of a prior art light emitting diode (hereinafter, referred to as LED) capable of suppressing the sub peak in the light emission intensity, disclosed in Japanese Published Patent Application 63-240083. In FIG. 12, the same reference numerals as those of FIG. 10 designate the same or corresponding portions. Reference numeral 102 designates a non-doped GaAs layer of less than 1.times.10.sup.16 cm.sup.-3 dopant concentration, disposed between the n-GaAs substrate 1 and the n-AlGaAs cladding layer 2. FIG. 13 is a diagram showing the relation between the dopant concentration of the GaAs layer and the intensity of PL light.
As shown in FIG. 13, the PL intensity of the GaAs layer of less than 1.times.10.sup.16 cm.sup.-3 dopant concentration is less than a hundredth of that of the GaAs layer of approximately 1.about.3.times.10.sup.18 cm.sup.-3 dopant concentration, which is generally used as a substrate. In this prior art LED, the non-doped GaAs layer 102 of less than 1.times.10.sup.16 cm.sup.-3 dopant concentration is provided between the n-GaAs substrate 1 and the n-AlGaAs cladding layer 2 and almost all of the light generated in the active layer 3 and spreading in a direction toward the substrate 1 is dissipated by recombinations which do not contribute to the light emission in the non-doped GaAs layer 100, thereby suppressing the sub peak in the light emission intensity.
The prior art LED that suppresses the sub peak in the light emission intensity constituted as described above has the following problems.
Firstly, at the time of growing the non-doped layer, a low dopant concentration layer can be produced in a clean growth equipment. However, it cannot be produced when the growth equipment is contaminated by the impurities from repeated growths. Therefore, the inside of the growth equipment must be always kept clean, thereby resulting in low productivity.
Secondly, even when the non-doped layer is produced in an epitaxial growth process, the impurities diffuse from the n-GaAs substrate of 1.about.3.times.10.sup.18 cm.sup.-3 dopant concentration and the n-AlGaAs cladding layer of 1.about.5.times.10.sup.17 cm.sup.-3 dopant concentration to the non-doped layer in the following annealing process such as a Zn diffusion process, and thus the dopant concentration of the non-doped layer is raised. Therefore, the PL intensity increases and the light emission intensity has a sub peak.
Thirdly, the insertion of the non-doped layer increases the series resistance of the element and this increases the power consumption. Thus, element deterioration is accelerated and lifetime is shortened.